Micromirror array device and a method for making the same

ABSTRACT

The spatial light modulator of the present invention comprises an array of micromirrors, each of which has a reflective deflectable mirror plate. A set of posts are provided for holding the mirror plates on a substrate, but not all micromirrors of the micromirror array have posts.

TECHNICAL FIELD OF THE INVENTION

The present invention is generally related to the art ofmicroelectromechanical devices, and more particularly, to micromirrorarray devices for use in display systems.

BACKGROUND OF THE INVENTION

Micromirror arrays are key components of microelectromechanical system(MEMS)-based spatial light modulators (SLMs). SLMs are transducers thatmodulate an incident beam of light in a spatial pattern that correspondsto an optical or electrical input. A typical MEMS-based SLM consists ofan array of individually addressable micromirrors. Each micromirrorconsists of a deflectable reflective mirror plate that is attached to adeformable hinge formed on a substrate such that the mirror plate canrotate on the substrate. Each individual mirror plate can be deflectedindependently by an electrostatic force. The electrostatic force isderived from an electrostatic field established between the mirror plateand an electrode. Reflection of a beam of incident light incident on amicromirror can then be controlled, for example, by deflecting themicromirror through changing the electrostatic force applied to themicromirror. MEMS-based SLM have experienced significant developmentsand have been implemented in many applications, one of which is the usein digital display systems. In a display application, each micromirroris associated with a pixel of a displayed image. To produce a brightpixel, the state of the micromirror associated with the pixel is set insuch a way that the reflected light from the micromirror is directedonto a target for viewing. To produce a dark pixel, the state of themicromirror is tuned such that the reflected light from the micromirroris directed away from the display device. In order to display ablack-and-white image, the micromirror array is illuminated by a beam oflight. By coordinating the reflective status of the micromirrors basedon the brightness of the pixels of the desired image, the collectiveeffect of all reflected lights from individual micromirrors is thegeneration of the desired image. Gray-scaled and colored images can alsobe displayed using the micromirror array with the pulse-width-modulationand sequential-color display techniques, which will not be discussed indetail herein.

Variations of the micromirror array, of which the SLM is comprised, havebeen developed. Regardless of the variations, the micromirror is the keycomponent of an SLM used for display systems and the quality of adisplayed image depends on the integrity and function of thatmicromirror. Therefore, a method and device that will simplify theproduct and the fabrication thereof is needed.

SUMMARY OF THE INVENTION

The objects and advantages of the present invention will be obvious, andin part appear hereafter and are accomplished by the present inventionthat provides a method and apparatus for operating pixels of spatiallight modulators in display systems. Such objects of the invention areachieved in the features of the independent claims attached hereto.Preferred embodiments are characterized in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

While the appended claims set forth the features of the presentinvention with particularity, the invention, together with its objectsand advantages, may be best understood from the following detaileddescription taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 illustrates an exemplary display system having a spatial lightmodulator in which embodiments of the invention can be implemented;

FIG. 2 a illustrates a perspective view of a portion of an exemplaryspatial light modulator in FIG. 1;

FIG. 2 b illustrates a cross-section view of another spatial lightmodulator in FIG. 2 a;

FIG. 3 illustrates a perspective view of another exemplary spatial lightmodulator according to an embodiment of the invention;

FIG. 4 illustrates an exemplary micromirror;

FIG. 5 illustrates another exemplary micromirror; and

FIG. 6 a to 6 c illustrates cross-section views of the micromirror inFIG. 2 b during an exemplary fabrication process.

DETAILED DESCRIPTION OF THE INVENTION

In the micromirror array device of the present invention, posts areprovided for selected micromirrors in the array, whereas all deflectablemirror plates of the micromirror array are supported and held by theprovided posts such that the mirror plates can be individually addressedand deflected on a substrate. For this purpose, hinges of themicromirrors are interconnected according to a particular pattern. Theposts are distributed across the array of micromirrors and connectingthe hinges to the substrate. The locations of the posts can bedetermined based on the connection pattern of the hinges, as well as themechanical properties of the hinges and/or the mechanical properties ofthe hinge connections. As a result, one or more micromirrors in themicromirror array are not provided with a post. The mirror plate and thehinge of such a micromirror are held by the other hinges and the posts.Meanwhile, there can be a micromirror in the micromirror array having atmost one post directly connected thereto.

The following description refers to drawings which are based on selectedexamples for demonstration purposes only and should not be interpretedas a limitation to the present invention. Other variations withoutdeparture from the spirit of the present invention are also applicable.

The micromirror array device of the present invention has a variety ofapplications, one of which is in display systems. FIG. 1 presents anexemplary display system that employs a spatial light modulatorcomprising an array of micromirrors. In its basic configuration, displaysystem 100 comprises light source 102, optical devices (e.g. light pipe104, condensing lens 108 and projection lens 112), display target 114and spatial light modulator 110 that further comprises a plurality ofmicromirror devices (e.g. an array of micromirrors). Light source 102(e.g. an arc lamp) emits light through the light integrator/pipe 104,color wheel 106 and condensing lens 108 and onto spatial light modulator110. Though the color wheel is positioned after the light pipe in thisexample, the color wheel may be positioned before the light pipe aswell. The micromirrors of the spatial light modulator 110 areselectively actuated by a controller (e.g. as disclosed in U.S. Pat. No.6,388,661 issued May 14, 2002, incorporated herein by reference) so asto reflect—when in their “ON” position—the incident light intoprojection optics 112, resulting in an image on display target 112(screen, a viewer's eyes, a photosensitive material, etc.). Generally,more complex optical systems are often used, especially in displayingapplications for color images.

FIG. 2 a depicts an example of a micromirror array that can be used forspatial light modulator 110 of FIG. 1. For simplicity purposes, only 4×4micromirrors are presented. In general, the micromirror array may havemillions of micromirrors, especially in display systems. For example,the micromirror array may have 1024×1768, or 1280×1024, or 1600×1024 oreven larger numbers of micromirrors.

The micromirror array can be constructed having a pitch (thecenter-to-center distance between adjacent micromirrors) of 25micrometers or less, or 10.16 micrometers or less, or from 4.38 to 10.16micrometers. The gap between adjacent micromirrors is approximately of0.5 micrometers or less, or from 0.1 to 0.5 micrometer. And the mirrorplate of the micromirror has a dimension of from 20 micrometers to 10micrometers.

In the particular example shown in FIG. 2 a, the micromirror array isformed on substrate 201, which can be a light transmissive substrate,such as glass or quartz. Each micromirror comprises a mirror plate (e.g.mirror plate 210) and a hinge (e.g. hinge 206). The mirror plate isattached to the hinge such that the mirror plate can rotate relative tothe substrate. The hinge is held by a hinge support (e.g. hinge support204) on the substrate. The hinge supports of the micromirrors in thearray are interconnected along a direction of the micromirror array. Forexample, the hinge supports of micromirrors 210, 212, 214, and 216 alongdirection AA are interconnected—forming a hinge support strip (e.g.hinge support strip 209). The hinge support strip as illustrated in thefigure is along direction AA, which is a diagonal of the micromirrorarray. In other embodiments of the invention, the hinge support stripsof the micromirror array can be along any desired directions in theplane of the micromirror array. For example, the hinge support stripscan be along the edges or along a direction at an angle to the edges ofthe micromirror array. A micromirror array may have a plurality of hingesupport strips that are parallel to each other when viewed from the top,in plan view. Alternatively, the micromirror array may have hingesupport strips that are not parallel to each other, which is not shownin the figure.

FIG. 2 b is a view of a cross-section taken along direction AA of FIG. 2a. The mirror plates of the micromirrors are attached to hinges that areheld by hinge supports. For example, mirror plate 210 is attached to adeformable hinge via hinge contact 207. The hinge is held by hingestructure 209 in FIG. 2 a. The hinge supports of micromirrors 210, 212,214, and 216 are interconnected to each other, forming hinge supportstrip 218. The hinge support strip is supported, held and connected tosubstrate 201 by posts 202 and 203. Posts 202 and 203 are connected toone end of the hinge structure of micromirror 211 and one end of thehinge structure of micromirror 217 respectively. However, the hingestructures of micromirrors 212 and 214 are not directly connected to apost. That is, the mirror plates 211 and 213 of micromirrors 212 and 214respectively are connected to the substrate via hinges and hingesupports, the hinge support strip and the posts that are connected tohinge supports of micromirrors other than micromirrors 212 and 214 (e.g.micromirrors 210 and 216). Of course, other arrangements for the postscan also be applied. In particular, posts can be provided for themicromirrors based upon the mechanical properties of the hinge supports.For example, fewer posts need be provided for micromirrors that haverigid hinges or hinge supports.

The hinge supports of the micromirrors in the micromirror array mayalternatively be interconnected to form a hinge support frame, as shownin FIG. 3. Referring to FIG. 3, the hinge supports of the micromirrorsare interconnected in both BB and CC directions—forming a hinge supportframe. The BB and CC directions may or may not be perpendicular to eachother. The BB and CC directions may or may not be parallel to the edgesof the micromirror array. With such a hinge support frame, themechanical stability and reliability of the micromirror array cancertainly be improved.

Though not shown in the figure, the hinge supports of the micromirrorsin the micromirror array can be interconnected into hinge-support stripsor hinge-support frames, or hinge-support grids, or a combination of thesame. The hinge support of a micromirror in an instance may stand alone,not being connected to other hinge supports.

The hinge support strips (frames and grids if any) are supported andheld by a plurality of posts on a substrate. The posts are positionedbetween the hinge support strips (frames or grids if any) on thesubstrate substrate, and connect the hinge support strips (frames orgrids if any) to the substrate. In an embodiment of the invention, eachpost is connected to a hinge support of a micromirror in the micromirrorarray. The post may be positioned at one or both ends along an axis of amirror plate to support the hinge support (i.e. hinge support 304 inFIG. 4 or hinge support 324 in FIG. 5). However, not all the individualhinge support structures of the micromirrors in the micromirror arrayare directly connected to one or two posts. For example, a hinge supportof a micromirror may have only one post connecting the hinge support to,and supporting the hinge support on, the substrate, or a micromirror mayhave no posts formed thereon. In this instance, the hinge and the hingesupport holding the hinge is supported by the hinge support strip(and/or the hinge support frame) to which the hinge support having nopost is connected.

According to the invention, posts are provided for the hinge supports ofselected micromirrors in a micromirror array only. Specifically, themicromirrors to be provided with posts can be selected according to apredetermined criterion. As a way of example, the criterion can be:posts are provided for every particular number m of micromirrors along agiven direction (e.g. a diagonal of the micromirror array or anydirection defined by the configuration of the hinge structures of themicromirrors); or posts are provided for micromirrors that are randomlyselected from the micromirror array; or posts are provided anddistributed based upon the mechanical properties of the hinge supportsand/or the hinge strips (or frames or grids if any).

As a way of example, the micromirror array may have a plurality of firstmicromirrors provided with at least two posts, and a plurality of secondmicromirrors provided with less than two posts, wherein the ratio of thenumber of first micromirrors to the second micromirrors is 1:2 or less,such as 1:3, or 1:4, or 1:5. As another example, the total number ofmicromirrors having at least two posts is approximately 90% or less,such as 80% or less, or 70% or less, or 60% or less, or 50% or less, oreven 40% or less, of the total number of the micromirrors in themicromirror array. As yet another example, the total number ofmicromirrors having no post can be approximately 5% or more, such as 10%or more, 30% or more, or 50% or more of the total number of micromirrorsin the micromirror array. Alternatively, the number of micromirrors eachhaving at least one post is approximately 90% or less, such as 80% orless, or 70% or less, or 50% or less, or 45% or less of the total numberof the micromirror provided with no post is approximately 90% or less,such as 80% or less, or 70% or less, or 50% or less, or 45% or less, or30% or less.

In another example, the micromirror array comprises a group of firstmicromirror devices each of which comprises a deflectable reflectivemirror plate, a deformable hinge, and at least two posts for holding themirror plate and hinge above a substrate; a group of second micromirrordevices each of which comprises a deflectable mirror plate, a deformablehinge, and at most one post for holding the mirror plate and hinge abovea substrate; wherein a ratio of the numbers of the first micromirrors tothe second micromirrors is 1:2 or less. The number of the firstmicromirrors can be 70% or less, such as 50% or less, 30% or less, ofthe total number of micromirrors in the micromirror array. The number ofthe second micromirrors can be 50% or more of the total number ofmicromirrors in the micromirror array. The group of the secondmicromirrors may further comprises a subgroup of third micromirrors eachof which having no post, and a subgroup of fourth micromirrors each ofwhich has a single post, wherein the number of the third micromirrors is10% or less of the total number of the second micromirrors in the group.The number of the third micromirrors can be 5% or less, such as 1% orless of the total number of the second micromirrors in the group. Thenumber of the fourth micromirrors can be 95% or more, 85% or more, 75%or more of the total number of the second micromirrors in the group.

In a micromirror array, the ratio of the total number of posts to thetotal number n of mirror plates is preferably less than (n+1)/n, such asless than (n+1)/2n.

In operation, illumination light is directed to the mirror plates of themicromirrors where the illumination is modulated so as to, for example,producing a desired image. The illumination light, however may bescattered by the portion of the posts exposed to the illumination light,generating undesired scattered light. In display applications, suchscattered light reduces the contrast ratio, thus degrading the qualityof the displayed images. For this reason, the posts, at least theportions exposed to the illumination light of the posts are coated witha light absorbing material.

The micromirrors of a micromirror array as those in FIGS. 2A and 3 mayhave a variety of configurations, one of which is shown in FIG. 4. Thisparticular example is shown with two posts 302, one on each end of ahinge support 304, which supports hinge 306. Hinge support 304 operateson a diagonal axis of the mirror plate of the micromirror. Its locationabove the central axis of the mirror plate allows it to rotateasymmetrically, that is, it is able to deflect to a larger angle in onedirection than another. Variations of stopping mechanisms may also beused to control the angle to which the mirror deflects, such as tip 310,in this example. FIG. 5 depicts yet another embodiment of a micromirror,in a different shape and structure. Differing from the mirror plate inFIG. 3 a, the mirror plate has jagged edges. One benefit of these jaggededges is that it decreases the amount of light scatter.

The micromirrors of the micromirror array each have a mirror plate and ahinge to which the mirror plate is attached such that the mirror platecan rotate relative to a substrate. The hinge can be formed underneaththe mirror plate in relation to the incident light. Specifically, themirror plate can be positioned between the hinge and the lighttransmissive substrate. This configuration has many advantages. Forexample, because the hinge is located underneath the mirror plate, it isnot exposed to the incident light. Therefore, unexpected lightscattering from the hinge can be avoided. Quality of the displayedimages, such as the contrast ratio of the displayed images can beimproved as compared to the micromirrors having exposed hinges.

In accordance with an embodiment of the invention, the mirror plate isattached to the hinge such that the mirror plate and the hinge arelocated in different planes parallel to the substrate and is operable torotate asymmetrically. Specifically, the mirror plate rotates to alarger angle (e.g. the ON state angle when operates in a binary statemode having the ON state and OFF state) in one direction than inanother. For this purpose, the mirror plate can be attached to the hingesuch that the attachment point is not at or around the geometric centerof the mirror plate. The rotation axis of the mirror plate can beparallel to but offset from a diagonal of the mirror plate when viewedfrom the top of the substrate on which the micromirror is formed. Ofcourse, other configurations wherein the mirror plates rotatesymmetrically are also applicable.

For individually deflecting the micromirrors of the micromirror array,an array of electrodes and circuitry can be provided with each electrodebeing associated with a micromirror. In operation, electrostatic fieldsare established between the mirror plates of the micromirrors and theelectrodes associated with the respective micromirrors, such that themirror plates can be deflected in response to electrostatic forcesderived from said electrostatic fields. In one embodiment, theelectrodes and circuitry can be formed on a semiconductor substrate thatis disposed at a specified distance proximate to the substrate on whichthe micromirrors are formed. Alternatively, the array of micromirrorsand the array of electrodes and circuitry can be formed on the samesubstrate, such as on a semiconductor substrate.

A demonstrative fabrication process for making the micromirror and themicromirror array of the present invention will be discussed in thefollowing with references to FIG. 6 a to FIG. 6 c. U.S. patentapplication Ser. No. 09/910,537 filed on Jul. 20, 2001, and U.S. patentapplication Ser. No. 60/300,533 filed on Jun. 22, 2001, both to Reid,contain examples of materials that may be used for the variouscomponents of the present invention. These patent applications are alsoincorporated herein by reference. The exemplary processes are fordemonstration purposes only and should not be interpreted as limitationsto the scope of the invention. In particular, although not limitedthereto, the exemplary micromirror is formed on a glass substrate thatis transparent to visible light, and electrodes and circuitry are formedon a separate substrate, such as a silicon wafer. However,alternatively, the micromirror and the electrodes and circuitry may beformed on the same substrate, without exceeding the scope of theinvention or altering the essence of the invention. For example, themicromirror substrate can be formed on a transfer substrate that islight transmissive. Specifically, the micromirror plate can be formed onthe transfer substrate and then the micromirror substrate along with thetransfer substrate is attached to another substrate such as a lighttransmissive substrate followed by removal of the transfer substrate andpatterning of the micromirror substrate to form the micromirror.

FIG. 6 a illustrates a cross-section view of the exemplary micromirrorarray in FIG. 2 b. The micromirror array is formed on substrate 400,which is transparent to visible light (e.g. glass, 1737F, Eagle 2000,quartz, Pyrex™, and sapphire). First sacrificial layer 402 is depositedon substrate 400. First sacrificial layer 402 may be any suitablematerial, such as amorphous silicon, or could alternatively be a polymeror polymide, or even polysilicon, silicon nitride, silicon dioxide andtungsten, depending upon the choice of sacrificial materials, and theetchant selected. In the embodiment of the invention, the firstsacrificial layer is amorphous silicon, and it is preferably depositedat 300–350° C. The thickness of the first sacrificial layer can bewide-ranging depending upon the size of the micromirror and the desiredtilt angle of the micromirror, though preferred is a thickness of from500 Å to 50,000 Å, preferably close to 25,000 Å. The first sacrificiallayer may be deposited on the substrate using any suitable method, suchas LPCVD or PECVD.

As an optional feature of the embodiment, an anti-reflection film may bedeposited on the surface of substrate 400. The anti-reflection film isdeposited for reducing the reflection of the incident light from thesurface of the substrate. Of course, other optical enhancing films maybe deposited on either surface of the glass substrate, as desired. Inaddition to the optical enhancing films, an electrode may be formed on asurface of substrate 400. The electrode can be formed as an electrodegrid or a series of electrode segments (e.g. electrode strips) aroundthe mirror plate. Alternatively, the electrode can be formed as anelectrode film on the surface of substrate 400, in which case, theelectrode film is transparent to visible light. The electrode can beused for driving the mirror plate to either the ON state or the OFFstate. Alternatively, a light-absorbing grid can be deposited on asurface of the glass substrate and around or below each micromirror. Thelight-absorbing frame absorbs incident light that reaches the substrateand/or light scattered from the edges of the micromirrors. By preventingunwanted reflection of light off the substrate, the absorption of thescattered light improves the quality of performance, such as thecontrast ratio, of the micromirror.

After depositing the first sacrificial layer, the mirror plates ofmicromirrors, such as mirror plates 404, 414, 416 and 418, are formed,the hinge supports of which are interconnected, such as in a hingesupport strip in this example or alternatively a frame. The mirrorplates are deposited and patterned on the first sacrificial layer.Because the micromirrors are designated for reflecting incident light inthe spectrum of interest (e.g. the visible light spectrum), it ispreferred that the micromirror plate layer comprises one or morematerials that exhibit high reflectivity (preferably 90% or higher) tothe incident light. The thickness of the micromirror plates can bewide-ranging, depending upon the desired mechanical properties (e.g. theelastic module), the size of the micromirror, the desired ON state angleand OFF state angle, the electronic properties (e.g. the conductivity)of the mirror plates and the properties of the materials selected forforming the micromirror plates. According to the invention, a thicknessfrom 500 Å to 50,0000 Å, preferably around 2,500 Å, is preferred for themirror plates. In the given embodiment of the invention, the mirrorplates are a multi-layered structure, which comprises a SiO_(x) layerwith a preferred thickness around 400 Å, a light-reflecting layer ofaluminum with a preferred thickness around 2,500 Å, a titanium layerwith a preferred thickness around 80 Å, and a 200 Å TiN_(x) layer. Inaddition to aluminum, other materials, such as Ti, AlSiCu and TiAl,having high reflectivity to visible light may also be used for thelight-reflecting layer. These mirror plate layers can be deposited byPVD at a temperature preferably around 150° C.

After deposition, mirror plates 404, 414, 416, and 418 are patternedinto desired shapes, such as the shapes depicted in FIG. 3 a or FIG. 3b. The patterning of the micromirrors can be achieved using standardphotoresist patterning followed by etching, using for example CF₄, Cl₂,or other suitable etchant depending upon the specific material of themicromirror plate layer.

After patterning the mirror plates, second sacrificial layer 412 isdeposited on first sacrificial layer 402 and mirror plates 404, 414, 416and 418. The second sacrificial layer may comprise amorphous silicon, orcould alternatively comprise one or more of the various materialsmentioned above in reference to the first sacrificial layer. First andsecond sacrificial layers need not be the same, although they are thesame in the preferred embodiment so that, in the future, the etchingprocess for removing these sacrificial materials can be simplified. Aswith the first sacrificial layer, the second sacrificial layer may bedeposited using any suitable method, such as LPCVD or PECVD. In theembodiment of the invention, the second sacrificial layer comprisesamorphous silicon deposited at approximately 350° C. The thickness ofthe second sacrificial layer can be on the order of 12,000 Å, but may beadjusted to any reasonable thickness, such as between 2,000 Å and 20,000Å, depending upon the desired distance (in the direction perpendicularto the micromirror plate and the substrate) between the micromirrorplate and the hinge. It is preferred that the hinge and mirror plate beseparated by a gap of a distance from 0.1 to 1.5 microns, morepreferably from 0.1 to 0.45 micron, and even more preferably from 0.25to 0.45 micron. Larger gaps could also be used, such as a gap from 0.5to 1.5 micrometers, or from 0.5 to 0.8 micrometer, or from 0.8 to 1.25micrometers, or from 1.25 to 1.5 micrometers.

In the preferred embodiment of the invention, the micromirror platescomprise aluminum, and the sacrificial layers (e.g. the first and secondsacrificial layer) are amorphous silicon. This design, however, cancause defects due to the diffusion of the aluminum and silicon,especially around the edge of the mirror plate. To solve this problem, aprotection layer (not shown) may be deposited on the patternedmicromirror plate before depositing the second sacrificial siliconlayer, such that the aluminum layer can be isolated from the siliconsacrificial layer. This protection may or may not be removed afterremoving the sacrificial materials. If the protection layer is not to beremoved, it is to be patterned, after deposition on the mirror plate.

The deposited second sacrificial layer is then patterned to form twodeep-via areas 406 and 420 (for the posts, such as posts 202 and 203respectively in FIG. 2 b) and shallow-via areas 410, 424, 426 and 428(for the hinge contacts, such as hinge contacts 207, 224, 226 and 228 inFIG. 2 b), using standard lithography technique followed by etching, asshown in the figure. For simplicity and demonstration purposes, theshallow-via areas may appear to be centered with respect to the mirrorplates; however, the shallow-via areas should be positioned, withrespect to the mirror plates, based on the design of the micromirror andthe location of the hinge contact thereof. For example, based on theembodiment of the micromirror array in FIG. 2 b, the shallow-via areaswould be located asymmetrically with respect to the center of the mirrorplates and should be formed as such. The etching step may be performedusing Cl₂, BCl₃, or other suitable etchant depending upon the specificmaterial(s) of the second sacrificial layer. The distance from onedeep-via area to the next depends upon the length of the defined hingesupport strip or pattern. Deep-via areas should not be formed betweentwo adjacent mirror plates of micromirrors connected to the hingesupport strip in the given embodiment, since there are no posts providedbetween two adjacent micromirrors, only one post at each end of thehinge support strip to which the micromirrors are connected. Forsimplicity and demonstration purposes, only 4 micromirrors are shown inthe cross-section of this fabrication process in FIG. 4 a; however, thedeep-via areas should be positioned, with respect to the micromirrorsand with one another, based on the design of the micromirror array andthe pattern of posts provided to support the hinge supports of themicromirrors. In order to form the shallow-via area, an etching stepusing CF₄ or other suitable etchant may be executed. The shallow-viaarea, which can be of any suitable size, is preferably on the order of2.2 square microns in area in cross-section, in the plane parallel tothe mirror plate and substrate in the embodiment, and the size of eachdeep-via is approximately 1.0 square micron in area in cross-section, inthe plane parallel to the mirror plate and substrate in the embodiment.

After patterning the second sacrificial layer, hinge support layer 408is deposited on the patterned second sacrificial layer. Because thehinge support is designated for holding the hinge and the micromirrorplate, it is desired that the hinge support layer comprises materialshaving a large elastic modulus. According to an embodiment of theinvention, hinge support layer 408 comprises a TiN_(x) layer of athickness of 400 Å (although it may comprise TiN_(x), and may have athickness between 100 Å and 2,000 Å) deposited by PVD, and a SiN_(x)layer of a thickness of 3500 Å (although the thickness of the SiN_(x)layer may be between 2,000 Å and 10,000 Å) deposited by PECVD. Ofcourse, other suitable materials and methods of deposition may be used(e.g. LPCVD or sputtering.) The TiN_(x) layer is not necessary for theinvention, but it provides a conductive contact surface between themicromirror and the hinge in order to reduce charge-induced stiction.

After the deposition, hinge support layer 408 is patterned into adesired configuration, such as that of hinge support strip 209 in FIG. 2a, or that of hinge support frame 252 in FIG. 3 or any other number ofpossible configurations, not specifically listed herein. An etching stepusing one or more proper etchants is executed in patterning the hingesupport layer. In particular, the layer can be etched with a chlorinechemistry or a fluorine chemistry where the etchant is a perfluorocarbonor hydrofluorocarbon or SF₆ that is energized so as to etch selectivelythe hinge support layers both chemically and physically (e.g. aplasma/RIE etch with CF₄, CHF₃, C₃F₈, CH₂F₂, C₂F₆, SF₆, etc. or morelikely combinations of the above or with additional gases, such asCF₄/H₂, SF₆/Cl₂, or gases using more than one etching species such asCF₂Cl₂, all possibly with one or more optional inert diluents.)Different etchants may, of course, be employed for etching each hingesupport layer (e.g. chlorine chemistry for a metal layer, hydrocarbon,or fluorocarbon or SF₆ plasma for silicon or silicon compound layers,etc.)

Referring to FIG. 6 b, after patterning the hinge support layer, thebottom segment of contact areas, such as contact area 506, is removedand a part of the micromirror plate underneath the contact area is thusexposed to hinge layer 504, which is deposited on the patterned hingesupport layer, to form an electric contact with external electricsource. The sidewalls of contact area 506 are left with residues of thehinge support layers after patterning. The residue on the sidewallshelps to enhance the mechanical and electrical properties of the hinge.When selected adjacent micromirrors do share a post, a deep-via area ofone micromirror can form a continuous element with a deep-via areacorresponding to the adjacent micromirror in an array. Only one deep-viaarea would need to be formed between said micromirrors. Of course, wherethere is no post, no deep-via area would be formed.

In the embodiment of the invention, the hinge layer is also used as anelectric contact for the micromirror plate. It is desired that thematerial of the hinge layer is electrically conductive. Examples ofsuitable materials for the hinge layer are silicon nitride, siliconoxide, silicon carbide, polysilicon, Al, Ir, titanium, titanium nitride,titanium oxide(s), titanium carbide, CoSiN_(x), TiSiN_(x), TaSiN_(x), orother ternary and higher compounds. When titanium is selected for thehinge layer, it can be deposited at 100° C. Alternatively the hingelayer may comprise multi-layers, such as 100 Å of TiN_(x) and 400 Å ofSiN_(x).

After deposition, the hinge layer is then patterned as desired usingetching. As with the hinge support layer, the hinge layer can be etchedwith a chlorine chemistry or a fluorine chemistry where the etchant is aperfluorocarbon or hydrofluorocarbon or SF₆ that is energized so as toetch selectively the hinge layers both chemically and physically (e.g. aplasma/RIE etch with CF₄, CHF₃, C₃F₈, CH₂F₂, C₂F₆, SF₆, etc. or morelikely combinations of the above or with additional gases, such asCF₄/H₂, SF₆/Cl₂, or gases using more than one etching species such asCF₂Cl₂, all possibly with one or more optional inert diluents.)Different etchants may, of course, be employed for etching each hingelayer (e.g. chlorine chemistry for a metal layer, hydrocarbon orfluorocarbon or SF₆ plasma for silicon or silicon compound layers, etc.)

After the hinge is formed, the micromirror is released by removing thesacrificial materials of the first and second sacrificial layers. A viewof the cross-section of the released micromirror device is presented inFIG. 6 c. The removal of the first and second sacrificial layers and thefollowing release of the micromirror device permit the mirror plate todeflect, when so driven by the electrodes.

In order to remove efficiently the sacrificial material (e.g. amorphoussilicon), the release etching utilizes an etchant gas capable ofspontaneous chemical etching of the sacrificial material, preferablyisotropic etching that chemically (and not physically) removes thesacrificial material. Such chemical etching and apparatus for performingsuch chemical etching are disclosed in U.S. patent application Ser. No.09/427,841 to Patel et al. filed Oct. 26, 1999, and in U.S. patentapplication Ser. No. 09/649,569 to Patel et al. filed Aug. 28, 2000, thesubject matter of each being incorporated herein by reference. Preferredetchants for the release etch are gas phase fluoride etchants that,except for the optional application of temperature, are not energized.Examples include HF gas, noble gas halides such as xenon difluoride, andinterhalogens such as IF₅, BrCl₃, BrF₃, IF₇ and ClF₃. The release etchmay comprise inner gas components such as N₂, Ar, Xe, He, etc. In thisway, the remaining sacrificial material is removed and themicromechanical structure is released. In one aspect of such anembodiment, XeF₂ is provided in an etching chamber with diluents (e.g.N₂ and He.) The concentration of XeF₂ is preferably 8 Torr, although theconcentration can be varied from 1 Torr to 30 Torr or higher. Thisnon-plasma etch is employed for preferably 900 seconds, although thetime can vary from 60 to 5000 seconds, depending on temperature, etchantconcentration, pressure, quantity of sacrificial material to be removed,or other factors. The etch rate may be held constant at 18 Å/s/Torr,although the etch rate may vary from 1 Å/s/Torr to 100 Å/s/Torr. Eachstep of the release process can be performed at room temperature.

In addition to the above etchants and etching methods mentioned for usein either the final release or in an intermediate etching step, thereare others that may also be used by themselves or in combination. Someof these include wet etches, such ACT, KOH, TMAH, HF (liquid); oxygenplasma, SCCO₂, or supercritical CO₂ (the use of supercritical CO₂ as anetchant is described in U.S. patent application Ser. No. 10/167,272,which is incorporated herein by reference.) However, spontaneous vaporphase chemical etchants are more preferred because the sacrificialmaterial, such as amorphous silicon, can be removed more efficiently insmall gaps between adjacent mirror plates and the lateral gap betweenlayers, in comparison to the efficiency of the removal of othersacrificial materials (e.g. organic materials) via other etchingmethods. Though said etching method is not required in all embodimentsof the present invention, a micromirror array with very small gaps, asmall pitch, and a small distance between the hinge and the mirror platecan be more easily fabricated with such spontaneous vapor phase chemicaletchants.

It will be appreciated by those skilled in the art that a newmicromirror array device has been described herein. In view of the manypossible embodiments to which the principles of this invention may beapplied, however, it should be recognized that the embodiments describedherein with respect to the drawing figures are meant to be illustrativeonly and should not be taken as limiting the scope of the invention. Forexample, those of skill in the art will recognize that the illustratedembodiments can be modified in arrangement and detail without departingfrom the spirit of the invention. In particular, other protectivematerials, such as inert gas, may be filled in the space formed by thepackage substrate and the cover substrate. Therefore, the invention asdescribed herein contemplates such embodiments as may come within thescope of the following claims and equivalents thereof. In the claims,only elements denoted by the words “means for” are intended to beinterpreted as means plus function claims under 35 U.S.C. §112, thesixth paragraph.

1. A projection system, comprising: an illumination system providing alight beam to be incident on a spatial light modulator; the spatiallight modulator that comprises: an array of micromirrors, eachmicromirror positioned at a pixel location, further comprising: a firstmicromirror at a first pixel location comprising: a deflectablereflective mirror plate attached to a deformable hinge that is supportedby a post on a substrate at the first pixel location; and a secondmicromirror at a second pixel location comprising: a deflectablereflective mirror plate attached to a deformable hinge that is notsupported by a post on the substrate at the second pixel location; andan optical element for directing light to or from the spatial lightmodulator.
 2. The projection system of claim 1, further comprising adisplay target.
 3. The projection system of claim 1, comprising a firstoptical element for directing the light beam to the spatial lightmodulator; and comprising a second optical element for projecting thelight beam from the spatial light modulator onto a target.
 4. Theprojection system of claim 1, having a resolution of 1024×768 or higher.5. The projection system of claim 1, wherein a plurality of firstmicromirrors are provided with at least two posts and a plurality ofsecond micromirrors are provided with less than two posts, wherein theratio of the number of first micromirrors to the total number ofmicromirrors is 1:2 or less.
 6. The projection system of claim 1,wherein a plurality of third micromirrors are provided with at least onepost and a plurality of forth micromirrors are not provided with a post,wherein the ratio of the number of first micromirrors to the totalnumber of micromirrors is 1:3 or less.
 7. The projection system of claim6, wherein the ratio is 1:4 or less.
 8. The projection system of claim1, wherein the deformable hinge comprises TiN_(x) or TiSi_(x)N_(y). 9.The projection system of claim 8, wherein the deformable hinge comprisesSiN_(x).
 10. The projection system of claim 1, wherein the hinge is amultilayered structure.
 11. The projection system of claim 1, whereinthe mirror plate is a multilayered structure, comprising a conductinglayer.
 12. The projection system of claim 11, wherein the conductinglayer is aluminum.
 13. The projection system of claim 11, wherein themirror plate further comprises a ceramic layer that comprises SiO_(x).14. The display system of claim 1, wherein the illumination systemcomprises: a light source; a light pipe; and a color filter.
 15. Thedisplay system of claim 14, wherein the light pipe is positioned betweenthe light source and the color filter.
 16. The display system of claim14, wherein the color filter is positioned between the light source andthe light pipe.
 17. The display system of claim 14, wherein the colorfilter is a color wheel having primary colors.
 18. The display system ofclaim 1, wherein each micromirror of the micromirror array has a hingesupport by which the hinge of the micromirror is held.
 19. The displaysystem of claim 18, wherein the mirror plate and the hinge are inseparate planes parallel to the substrate.
 20. The display system ofclaim 18, wherein the hinge and the mirror plate are in the same planeparallel to the substrate.
 21. The display system of claim 1, whereinthe mirror plate is positioned between the hinge and the substrate ofthe micromirror.
 22. The display system of claim 18, wherein the hingesupports of a set of the array of micromirrors are interconnectedtogether forming a hinge support strip.
 23. The display system of claim18, wherein the hinge supports of a set of the array of micromirrors areinterconnected together forming a hinge support frame.
 24. The displaysystem of claim 18, wherein the hinge supports of a set of the array ofmicromirrors are interconnected together forming a hinge support grid.25. The display system of claim 1, wherein the mirror plate and/or hingeis made of single crystal silicon patterned from a silicon substrate.26. The display system of claim 1, wherein the post of the first mirrorplate comprises a light absorbing layer thereon for absorbing light fromthe illumination system that is incident thereon.
 27. The display systemof claim 1, wherein the first mirror plate is held by the post on asubstrate transmissive to visible light.
 28. The display system of claim1, wherein the first mirror plate is held by the post on an opaquesubstrate.
 29. The display system of claim 28, wherein the opaquesubstrate is a silicon substrate.
 30. The display system of claim 1,wherein the posts comprises a portion having a light absorbing materialcoated thereon for reducing light scattering.
 31. A projection system,comprising: an illumination system providing a light beam for thesystem; a spatial light modulator that comprises: an array ofmicromirrors, at least one of which comprises: a deflectable reflectivemirror plate attached to a deformable hinge that is directly supportedby a post on a substrate; and at least another one of which comprises: adeflectable reflective mirror plate attached to a deformable hinge thatis not directly supported by a post on the substrate; an optical elementfor directing light to or from the spatial light modulator.
 32. Aprojection system comprising: a light source for providing light onto anarray of micromirrors; an array of micromirrors comprising a first groupand a second group, wherein micromirrors of the first group each have amicromirror plate and two posts adjacent the micromirror plate fordeflectably supporting the micromirror plate, and wherein micromirrorsof the second group each have a micromirror plate and a single postadjacent the micromirror plate for deflectably supporting themicromirror plate.
 33. The projection system of claim 32, wherein thearray of micromirrors comprises a third group each having a micromirrorplate and no posts adjacent the micromirror plate, wherein the thirdgroup of micromirrors is connected to adjacent micromirrors for support.34. A display system, comprising: an illumination system; a spatiallight modulator that comprises: a substrate; an array of reflectivedeflectable mirror plates on the substrate; an array of hinges and hingesupports, each hinge being held by the hinge support and attached to amirror plate such that the mirror plate can rotate relative to thesubstrate; and wherein a first hinge support of said hinge supports hasconnected thereto a post connecting the first hinge support to thesubstrate, and wherein a second hinge support of said hinge supports hasno post directly connecting the second hinge support to the substrate;and an optical element for directing light to or from the spatial lightmodulator.
 35. The display system of claim 34, further comprising adisplay target.
 36. The display system of claim 34, wherein each postdirectly connects one of the array of hinge supports to the substrate,and wherein at least one of the array of hinge supports is not directlyconnected to a post.
 37. The display system of claim 34, wherein themicromirrors having posts connected thereto are located in themicromirror array complying with a predetermined pattern.
 38. Thedisplay system of claim 36, wherein the predetermined pattern comprisesthat a particular number of micromirrors having no posts connectedthereto is positioned between two consecutive micromirrors both havingat least one post directly connected thereto along a particulardirection.
 39. The display system of claim 38, wherein the particularnumber is a random number.
 40. The display system of claim 38, whereinthe particular number is determined based upon a mechanical property ofthe hinges supports and hinges.
 41. The display system of claim 34,wherein the illumination system comprises: a light source; a light pipe;and a color filter.
 42. The display system of claim 41, wherein thelight pipe is positioned between the light source and the color filter.43. The display system of claim 41, wherein the color filter ispositioned between the light source and the light pipe.
 44. The displaysystem of claim 41, wherein the color filter is a color wheel havingprimary colors.
 45. The display system of claim 34, wherein the mirrorplate and the hinge of the micromirror are in separate planes parallelto the substrate.
 46. The display system of claim 34, wherein the hingeand the mirror plate are in the same plane parallel to the substrate.47. The display system of claim 34, wherein the mirror plate ispositioned between the hinge and the substrate of the micromirror. 48.The display system of claim 34, wherein the hinge supports of a set ofthe array of micromirrors are interconnected together forming a hingesupport strip.
 49. The display system of claim 34, wherein the hingesupports of a set of the array of micromirrors are interconnectedtogether forming a hinge support frame.
 50. The display system of claim34, wherein the hinge supports of a set of the array of micromirrors areinterconnected together forming a hinge support grid.
 51. The projectionsystem of claim 34, having a resolution of 1024×768 or higher.
 52. Theprojection system of claim 34, wherein a plurality of first micromirrorsare provided with at least two posts and a plurality of secondmicromirrors are provided with less than two posts, wherein the ratio ofthe number of first micromirrors to the total number of micromirrors is1:2 or less.
 53. The projection system of claim 34, wherein a pluralityof third micromirrors are provided with at least one post and aplurality of forth micromirrors are not provided with a post, whereinthe ratio of the number of first micromirrors to the total number ofmicromirrors is 1:3 or less.
 54. The projection system of claim 53,wherein the ratio is 1:4 or less.
 55. The projection system of claim 34,wherein the hinge comprises TiN_(x).
 56. The projection system of claim34, wherein the deformable hinge comprises SiN_(x).
 57. The projectionsystem of claim 34, wherein the hinge is a multilayered structure. 58.The projection system of claim 57, wherein the mirror plate is amultilayered structure, comprising a conducting layer.
 59. Theprojection system of claim 58, wherein the conducting layer is aluminum.60. The projection system of claim 58, wherein the mirror plate furthercomprises a ceramic layer that comprises SiO_(x).
 61. A spatial lightmodulator, comprising: a substrate; an array of reflective deflectablemirror plates on the substrate; a hinge support frame composed of a setof hinge supports, each hinge support holding a deformable hinge towhich a mirror plate of the array of mirror plates is attached; and aplurality of posts positioned between the hinge frames and the substratefor supporting the hinge supports on the substrate such that the mirrorplate can be rotated on the substrate; wherein at least one hingesupport is directly connected to less than two posts.
 62. A projectionsystem, comprising: an illumination system; and a spatial lightmodulator comprising: a group of first micromirror devices each of whichcomprises a deflectable reflective mirror plate, a deformable hinge, andat least two posts for holding the mirror plate and hinge above asubstrate; and a group of second micromirror devices each of whichcomprises a deflectable mirror plate, a deformable hinge, and at mostone post for holding the mirror plate and hinge above a substrate;wherein a ratio of the numbers of the first micromirrors to the secondmicromirrors is 1:2 or less.
 63. The projection system of claim 62,wherein the number of the first micromirrors is 70% or less of the totalnumber of micromirrors in the micromirror array.
 64. The projectionsystem of claim 63, wherein the number of the first micromirrors is 50%or less of the total number of micromirrors in the micromirror array.65. The projection system of claim 63, wherein the number of the secondmicromirrors is 30% or more of the total number of micromirrors in themicromirror array.
 66. The projection system of claim 63, wherein thenumber of the second micromirrors is 50% or more of the total number ofmicromirrors in the micromirror array.
 67. The projection system ofclaim 62, wherein the group of second micromirrors further comprises asubgroup of third micromirrors each of which having no post, and asubgroup of fourth micromirrors each of which has a single post, whereinthe number of the third micromirrors is 10% or less of the total numberof the second micromirrors in the group.
 68. The projection system ofclaim 67, wherein the number of the third micromirrors is 5% or less ofthe total number of the second micromirrors in the group.
 69. Theprojection system of claim 67, wherein the number of the thirdmicromirrors is 1% or less of the total number of the secondmicromirrors in the group.
 70. The projection system of claim 67,wherein the number of the fourth micromirrors is 95% or more of thetotal number of the second micromirrors in the group.
 71. The projectionsystem of claim 67, wherein the number of the fourth micromirrors is 85%or more of the total number of the second micromirrors in the group. 72.The projection system of claim 67, wherein the number of the fourthmicromirrors is 75% or more of the total number of the secondmicromirrors in the group.
 73. A method of making a spatial lightmodulator, comprising: depositing a first and second sacrificial layeron a substrate; forming a array of mirror plates and a plurality ofposts for the mirror plates on the first and second sacrificial layer,wherein a ratio of the number of the plurality of posts to the totalnumber n of mirror plates is less than (n+1)/n; forming a hinge for eachmirror plate; and removing the first and second sacrificial layer so asto release the mirror plates.
 74. The method of claim 73, wherein thefirst and second sacrificial layer comprises amorphous silicon; andwherein the step of removing the first and second sacrificial layerfurther comprising: removing the amorphous silicon with a vapor phaseetchant.
 75. The method of claim 74, wherein the etchant is anspontaneous vapor phase etchant.
 76. The method of claim 75, wherein theetchant is an interhalogen.
 77. The method of claim 75, wherein theetchant is a noble gas halide.
 78. The method of claim 75, wherein theetchant is HF.
 79. The method of claim 73, wherein the ratio is lessthan (n+1)/2n.